1. Field of the Invention
This invention relates to a plasma addressing electro-optical device having a two-layer structure including two layers of an electro-optical cell such as a liquid crystal cell and a plasma cell, and more particularly to the structure of a discharge electrode provided in a plasma cell in a plasma addressing electro-optical device of the type mentioned.
2. Description of the Related Art
An electro-optical device of the matrix type which employs a liquid crystal cell as an electro-optical cell such as, for example, a liquid crystal display device, conventionally employs, as a commonly known means for assuring a high resolution and a high contrast, an active matrix addressing system wherein a switching element such as a thin film transistor is provided for each picture element and the switching elements are driven in a line sequential condition. However, according to the active matrix addressing system, it is necessary to provide a large number of semiconductor elements such as thin film transistors on a substrate. Accordingly, the active matrix addressing system is disadvantageous in that, when the substrate has a large area, the yield in production is low.
A solution to the disadvantage has been proposed by Buzak et al. and is disclosed in Japanese Patent Laid-Open Application No. Heisei 1-217396 (corresponding to U.S. Pat. No. 4,896,149 and No. 5,077,553) wherein a plasma switch is employed in place of a switching element formed from a thin film transistor or a like element. The, general construction of a plasma addressing display device wherein a liquid crystal cell is driven making use of switches based on plasma discharge will be briefly described. Referring to FIG. 3, the plasma addressing display device is shown which has a layered flat panel structure which includes a liquid crystal cell 101, a plasma cell 102 and a common intermediate sheet 103 interposed between the liquid crystal cell 101 and the plasma cell 102. The plasma cell 102 is formed by using a glass substrate 104 and has a plurality of parallel channels 105 formed on a surface thereof. The channels 105 extend, for example, in a direction along the rows of the matrix. The channels 105 are individually closed by the intermediate sheet 103 so as to define plasma chambers 106 which are individually separated from each other. Ionizable gas is enclosed in each of the plasma chambers 106. An extending portion 107 of the glass substrate 104 is disposed between each adjacent ones of the channels 105 and serves as a barrier rib for isolating the adjacent plasma chambers 106 from each other and also as a gap spacer for the plasma chambers 106. A pair of parallel plasma electrodes are provided on a curved bottom surface of each of the channels 105 and function as an anode A and as cathode K so as to ionize the gas in the plasma chamber 106 to produce discharge plasma. Such discharge area forms a row scanning unit.
The the liquid crystal cell 101 is constructed using a transparent substrate 108. The substrate 108 is disposed in an opposing relationship to the intermediate sheet 103 with a predetermined gap left therebetween, and a liquid crystal layer 109 fills the gap. Signal electrodes 110 made of a transparent conducting material are formed on an inner surface of the substrate 108. The signal electrodes 110 extend perpendicularly to the plasma chambers 106 and form column driving units. Picture elements in a matrix are defined at intersecting positions between the column driving units and the row scanning units.
In the display device having such a construction as described above, the plasma chambers 106 in which plasma discharge occur are switched so as to be scanned in a line sequential condition when an analog driving voltage is applied to the signal electrodes 110 of the liquid crystal cell side in synchronism with such scanning signal so as to effect driving of the display apparatus. If plasma discharge occurs in a plasma chamber 106, the potential of the inside of the plasma chamber 106 becomes substantially uniform to that of the anode A so that picture element selection of each row is performed. In other words, each of the plasma chamber 106 functions as a sampling switch. If a driving voltage is applied to each picture element while the plasma sampling switch is in an on state, then sample hold is performed so that the lighting or extinction of the picture element can be controlled. Also after the plasma sampling switch is placed into an off condition, the analog driving voltage is held to a value of the picture element.
In the structure shown in FIG. 3, the plasma electrodes are formed on the curved bottom surfaces of the channels 105 such that an anode A and a cathode K in each pair are disposed in an opposing relationship in an inclined condition. Such arrangement is hereinafter referred to as inclined surface electrode structure. In this structure, the path of plasma discharge is formed between the electrode surface of the anode A to the electrode surface of the opposing cathode K, and accordingly, stable and efficient plasma discharge can be obtained.
However, in order to realize such inclined surface electrode structure as described above, it is necessary to form a stripe-like groove 105 on the surface of the substrate 104, but this involves considerable difficulty in production, and particularly it is very difficult to provide a stripe pattern with a high density. Also, it is complicated and difficult to actually form plasma electrodes in individual channels with an etching process.
So as to solve such problems of the conventional plasma addressing electro-optical devices to be solved into consideration, the applicant of the present invention has proposed, in Japanese Patent Application No. Heisei 3-47784 previously filed, a plasma addressing electro-optical device which is simple in structure and is suitable to be used to produce a screen having a large size and/or a high resolution. The structure of the proposed device will be described subsequently with reference to FIG. 4 to facilitate understanding of the present invention. Also the proposed device has a flat panel structure wherein a liquid crystal cell 201 and a plasma cell 202 are layered with an intermediate sheet 203 interposed therebetween. The liquid crystal cell 201 has basically the same structure as the liquid crystal cell 101 shown in FIG. 1. Ionizable gas is enclosed in a plasma chamber 205 defined between the intermediate sheet 203 and a lower substrate 204. A plurality of stripe-like plasma electrodes 206 are formed in a predetermined spaced relationship from each other on an inner surface of the substrate 204. Since the plasma electrodes 206 can be formed on a flat substrate by screen printing or a like technique, the production rate is high and the plasma electrodes 206 can be finely accurately formed. A barrier rib 207 is formed on each of the plasma electrodes 206, and the barrier ribs 207 divide the plasma chamber 205 into several discharge regions which form row scanning units. Also the barrier ribs 207 can be formed by screen printing or a like technique.
The plasma electrodes 206 are formed on the plane of the substrate 204, and since the discharge surfaces of the plasma electrodes 206 are positioned at the same level, they are not opposed to each other which is, different from those in the inclined surface electrode structure described hereinabove. The electrode arrangement shown in FIG. 4 will be hereinafter referred to as a flat surface electrode structure. Apparently the flat surface electrode structure has several significant advantages during manufacture as compared with the inclined surface electrode structure. Further, all of the barrier ribs 207 are formed on the plasma electrodes 206, and the anodes A and the cathodes K have the same construction. Accordingly, when the device shown in FIG. 4 is driven, the plasma electrodes 206 alternately change over in function between anodes A and cathodes K. Consequently, the plasma electrodes 206 are used as both as of the anodes and as the cathodes, and accordingly in, there is an advantage that the number of effective scanning lines can be doubled as compared with that of the structure shown in FIG. 3.
However, the flat surface electrode structure described above has several problems which are to be solved. First, in order to produce stable plasma discharge, the space or distance of the plasma cell 202, that is, the height of the plasma chamber 205, must necessarily be set to be equal to or larger than about 75% of the distance between the anode/cathode electrodes, which is a restriction on the structure of the cell. In particular, according to the flat surface electrode structure, the electrode surfaces of the anodes/cathodes are not opposed to each other, and paths of plasma discharge form substantially parabolic curves. If the height of the plasma chamber 205 is small, the discharge paths are intercepted by the intermediate sheet 203 constituting the ceiling of the plasma chamber 205, and consequently, stable plasma discharge cannot be maintained. As the distance between the anodes and the cathodes increases, the height of the plasma chamber must be increased, making it difficult to manufacture.
The second problem is the displacement in pitch between the plasma electrodes 206 and the barrier ribs 207. Both both members are formed by screen printing or a like technique, and as the stripe pattern becomes finer, the error in alignment become more significant so that the barrier ribs 207 may be placed out of register with the plasma electrodes 206. Consequently, the areas of the exposed surfaces of the plasma electrodes 206 on the opposite sides of each barrier rib 207 become different from each other. On the side on which the exposed surface area decreases, only a side end portion of the electrode contributes to plasma discharge, and as a result of concentration of the electric field, a breakdown is liable to occur which causes a failure in discharge.
The third problem is that plasma discharge becomes unstable form a barrier rib positioned on an anode. It is believed that the reason for this that the barrier rib is made of a dielectric material which acts to prevent electrons from flowing from a cathode to an anode. FIG. 5 shows simulated electron trajectories in a plasma cell. Referring to FIG. 5, each solid line indicates a trajectory in a complete vacuum and each broken line indicates an estimated electric line of force. The former corresponds to a case wherein the velocity component of the electron is at the maximum while the other corresponds to another case where the velocity component is lost completely due to collision with plasma and, accordingly, is equal to zero. Actually, electrons will travel intermediate trajectories between those indicated. Since the velocity component is not reduced to zero, an electron emitted by a cathode considerably overruns an anode. However, when stable plasma discharge is maintained, electron trajectories take discharge paths on the inner sides of the electrodes. Accordingly, it is believed that, above an anode, an electron flies to a position which is considerably inwardly from the end of the electrode. Consequently, if a barrier rib 207 of a dielectric material is present at a central portion of an anode electrode makes the exposed surface area of the electrode extremely small, then electrons are prevented from flowing into the electrode.